I Can’t Answer

This is distinct from thinking I don’t know or going blank. If you can’t answer then there is an underlying aspect of the question that means an answer can’t be given. There could be many possible reasons:

You might not be able to answer because you don’t have all of the information – so tell your examiners that.

You might not be able to answer because the question is not something you have investigated – so tell your examiners that.

You might not be able to answer because you decided that question was not worth following – so tell your examiners that – and tell them why.

You might not be able to answer their question fully or definitively, because it’s the sort of question to which one can only give an opinion, backed up with an argument and reasons – so do that.

The motivating question for this post is hypothetical. It may never come up in your viva. It’s probably better to invest time in things that you can control or influence. If a question does come up and you can’t answer: tell your examiners. Tell them why. Ask them questions.